Computed Tomography Physics, Instrumentation and Imaging
Module G
Required reading for this module is Chapters 11 and 13 of your text.
Seeram, Computed Tomography 2nd edition, Saunders/Elsevier.
The CT technologist plays an invaluable role in the diagnosis of disease by providing
the radiologist with quality CT images, through the careful application of theoretical
knowledge and clinical skills. As mentioned in the previous module (F), the
technologist controls both the scanning and reconstruction processes either through
manually selecting scan and reconstruction processes, or selecting the correct preprogrammed “protocols”. This module will initially provide information on these
“selectable scan factors”, and then discuss some of the image quality factors, such as
geometric and contrast resolution, presented in Module F.
SELECTABLE SCAN FACTORS
Let’s begin the discussion by explaining the importance of the scan field of view or SFOV.
The SFOV is the size of the field in the gantry aperture. It refers to the area of anatomical
interest. Remember that this is where data for image reconstruction is gathered. When
selecting this scan parameter, the technologist is essentially telling the detectors which
data to use and which data to ignore. The SFOV determines the number of detectors
required to collect data for a particular procedure, and should be larger that the area of
interest. The complete anatomical area must be included in the SFOV in order to avoid
the production of out-of-field artifacts. Out-of-field artifacts can present as streaking,
shading, or the miss-assignment of CT numbers. Any tissue located outside of the “field”
will not be used in the reconstruction process. CT scanners in use today, normally
provide pre-selected settings for such procedures as those for CT of the head (pediatric
head, small, medium, and large adult heads).
Body CT selections should be based on patient size and are usually regarded as the
following:
Small = 25 cm
Medium = 35 cm
Large = 48 cm
X-large = 50 cm
If the scanner has the choices listed above, the technologist should be especially
mindful to set the parameter, so as to include all of the patient’s anatomy without
setting the field either too small or too large. Too large a SFOV may also produces
artifacts, due to the fact that this can also cause shading and streaking at the skin
surface.
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Some scanners automatically determine the correct SFOV when the technologist
positions the lateral lines for the procedure. These settings include calibrations
specific to a particular portion of the anatomy. Thus it is very important that these
lines are set to include all of the lateral body contours.
Remember to select a scan-field-of view that optimizes image quality
and minimizes artifacts.
The display field of view (DFOV) is the next selectable scan factor to be discussed.
The DFOV, also referred to as the reconstructed field of view (RFOV), determines the
size of the image viewed on the monitor(s), reproduced, and communicated. The
DFOV plus the matrix size, determine the limitations of perceived detail. If selected
too large, it reduces the size of the image being displayed. The viewer’s retina senses
this, and the perception of detail is reduced. In actuality, the detail of the viewed
image is reduced because it is displayed using fewer pixels. The fewer pixels used for
display results in each of them being larger; the larger the pixel the less the detail. If
set too large, important anatomical information, especially of that located near the
skin surface, will be deleted.
If selecting both an SFOV and a DFOV, the DFOV must be equal to, or less than the
SFOV, never larger.
Modern CT scanners allow minimal DFOV selection to be prescribed from the CT
scout. The DFOV also impacts image noise and resolution. Wider DFOVs increase
the quantity of the photons from which data is retrieved. It reduces the amount of
noise present on the image, but at the expense of resolution. This is not the only
parameter that affects noise and resolution though. The focal spot size, scanner
geometry, detector size, and the reconstruction algorithm, etc. also affect noise and
resolution and will be discussed in greater detail later. The manufacturer’s and
operator’s goal should be to produce high quality images with excellent resolution
and limited noise. Moderate increases in mA usually reduce noise, but at the expense
of increased radiation dose to the patient.
Patient radiation dose is directly proportional to the mA (tube current). Reducing the
mA by half, results in decreasing the radiation dose by half. For example, at 100 mA,
the skin exposure is approximately 1.59R. Reducing the mA to 50, results in a skin
exposure of .79R.
Many of the scanners in use today have the ability to produce more than one set of
reconstructed images, using the same raw data. This allows the technologist to use
multiple combinations of DFOVs and convolution (reconstruction) filters to produce
extremely high quality images.
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Reconstruction filters;
are primarily responsible for ensuring that the scanned anatomy is accurately
represented on the final image
secondarily responsible for enhancing the spatial or contrast resolution of the final
image, depending on the anatomy scanned
if termed High-pass (sharp) filters, provide definitive borders and edges (resulting in
increased image noise) – used for high contrast areas – musculoskeletal
if termed Low-pass (soft) filters, do not define borders and edges to the extent of the
high-pass or sharp filter, so images do not generally exhibit noticeable amounts of
noise-used primarily for low contrast areas such as the brain and abdomen
The matrix size selection is also a selectable scan factor in most instances. The 512 x
512 matrix size is most common, some equipment allows for the selection of a 1024 x
1024 image matrix. In a 512 x 512 image matrix, there are 262,144 pixels, whereas in
a 1024 x 1024 matrix, there are 1,048, 76 pixels. So, you see, references to larger or
smaller matrices simply refer to the number of pixels contained within. Smaller
matrices, because they contain fewer pixels, reduce the memory requirements of the
scanner, allowing faster image reconstruction. Remember, the smaller the pixel, the
greater the detail.
Pixel size can be calculated for the variable matrices, using the formula:
Pixel size = DVOV / Matrix Size
Slice thickness is another selectable scan factor.
In conventional CT (slice-by-slice acquisition), slice thickness affects perceived
detail, because of the effect of volume-averaging for structures with complex shapes.
Thinner slices, because of the greater number used to image the anatomy of interest,
increase radiation dose to the patient.
They should only be used for their value in imaging:
1. areas with complex anatomy and high subject contrast such as the temporal bones
and orbits
2. the larynx parathyroid glands, and biliary system
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Remember, thin slices are many times quite “noisy”. To compensate for the increased
noise, tube output (mA) should be increased moderately. That is if the slices are acquired
in conventional fashion. In spiral CT, the selection of thin slices does not result in
increased radiation dose, since it is the volume of the area of interest that is acquired
during the scanning process, not the slices. The slices are generated by the computer as
specified by technologist or protocol selected. What does change increase radiation dose
in spiral scanning is pitch. When pitch is doubled, radiation dose is reduced by about
half. Changing pitch from 1:1 to 1.5:1 reduces exposure by about 33%.
KVp and mA are long-standing scan parameters selected by technologists. In CT, the
selectable kVp range is generally between 80 and 140, while mA selections are variable
depending on the scanner model and manufacturer. The signal to noise ratio may be
improved by increasing the kV, mA, or scan time. Normally only the mA is manipulated.
It may be noticed in the clinical setting, that some CT machines automatically increase or
decrease the mA, depending on the anatomy being scanned and the desired quality of the
image (signal to noise ratio).
The scan time, although selectable by the CT technologist, generally range between <1
second to 10 seconds. Short scan times reduce the effects of motion artifacts, and
intrinsic motion in the heart and bowel. Short scan times also contribute to patient
comfort. As you know, spiral CT with its short scan times, has been well received by both
the medical and patient communities.
The rate at which a CT machine samples data is selectable at purchase, but effect scan
time select ability. The types of sampling, angular and ray, were discussed in an earlier
module. A minimum number of samples are required to produce images free of the
sampling artifact, aliasing. All of these factors contribute to image quality in CT.
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IMAGE QUALITY
In Module F some of the factors that contribute to image quality were introduced.
These factors are inclusive of:
1. spatial resolution
2. contrast resolution
3. system noise
4. linearity
5. spatial uniformity
Spatial resolution is defined as the “degree of blurring in an image” and “the measure
of the ability of thee CT scanner to discriminate objects of varying densities located
close together, against a uniform background.” To image objects with excellent
spatial resolution depends on the slice thickness or section collimation. Spatial
resolution can be represented by the Point Spread Function (PSF), Line Spread
Function (LSF), and the Modulation Transfer Function (MSF). In CT, the MTF, a
mathematically complex formulation, and its associated graphic representations, is
generally used to express spatial resolution.
A simple definition of the MTF is the expression of the ratio of the fidelity of an
image to the original object scanned. An optimal fidelity image has an MTF value of
1, a completed non-optimal image has an MTF of 0, while the majority of CT images
fall somewhere between o and 1. The MTF in CT is measured using a line pair
phantom in which there are a series descending size bars. The phantom is scanned
and spatial resolution is reported in line pairs/mm (lp/mm). The spatial resolution
of the CT system is determined by the smallest line pair that can be resolved (seen)
on the resultant image. The spaces between the bars and the bars themselves, is
called the spatial frequency. The smaller the object, the more spatial frequency is
required for scanning. Currently, the optimal resolution that can be obtained from a
CT scanner is limited to scanning an object of about 0.3mm. Spatial resolution is also
effected by the geometric factors listed in Module F.
Contrast resolution, also sometimes referred to as low contrast or tissue resolution,
is defined as the ability of as CT scanner “to distinguish material of one composition
from another without regard for size or shape.” In other words, it is the ability of the
scanner to demonstrate small changes in tissue contrast. It has been reported that
this is an area where CT scanners excel, especially as compared to conventional
radiography. In CT, absorption of x-ray photons in body tissue is characterized by
Hounsfield Units or Linear Attenuation Coefficients (CT numbers), which are a
function of the energy of the photons themselves, as well as the atomic number of the
body tissue scanned, and its associated mass density.
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There is a limitation however.
The ability of a CT scanner to image low contrast objects is limited by the:

Size of the object
Uniformity of the object

System noise
Noise plays a significant role in image quality. It reduces resolution of low contrast
objects. In CT, noise is demonstrated on the images as “graininess”. Noise may also
be referred to as quantum mottle or quantum noise. Noise is representative of
deviations from the uniformity of the image matrix in which low contrast CT
numbers slightly above or below zero (0), are interpreted as 0. Another working
definition for noise is the one offered by Stewart Bushong. Dr, Bushong describes
noise as “the percent standard deviation of a large number of pixels obtained from a
water bath scan.”
Researchers agree that noise is dependant on not just one, but several factors,
including:
kVp and filtration
pixel size
slice thickness
detector efficiency
patient dose
Patient dose is the ultimate factor that controls noise. According to Bushong, system
noise can be defined using the following formula.
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CT image noise is one or the parameters that should be measured daily, usually by
the technologist. A 20 centimeter water bath phantom is normally used. Scanners are
equipped to identify a region of interest (ROI) and to compute the standard
deviation of CT numbers within the ROI. The technologist should position the ROI
to include a minimum of one hundred (100) pixels, and five determinants; four on
the periphery of the ROI and one in the center.
Another area where daily checks should be performed is linearity. Linearity is the
accuracy of the calibration for a CT scanner. In this area, a special phantom is
scanned to check that water measures zero (0). The phantom is called a five-pin
performance phantom. After scanning the phantom, the CT system records the
values in the phantom; the standard deviation is calculated and plotted.
The observed plotline of the CT number and the linear attenuation coefficient should
a straight line passing through zero, the CT number for water. This is the linearity,
and a deviation from linearity indicates that there is malfunction or misalignment of
the scanner. While minor deviations in linearity may not be visible on images, they
certainly affect the quantitative analysis of body tissue, since any deviation results in
inaccurate CT number generation.
Whenever a water bath CT phantom is scanned, the pixel values should be constant
in every region of the resultant image. This is termed spatial uniformity and can also
be measured or tested. Spatial uniformity is generally tested using an internal
software package. This software package allows the plotting of CT numbers along any
axis of the image. A histogram or line graph is generated. Acceptable spatial
uniformity is exhibited if the histogram or line graph value is within + or- 2 standard
deviations of the mean.
This module has presented information on “selectable scan factors” and image
quality. For further enumeration on these topics, please review the appropriate
sections in your text.
Module H will provide more information on conventional CT vs. single detector row,
and single detector row vs. multi-detector row CT.
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